The soybean cyst nematode (SCN) is the most economically important pathogen of soybean in the United States. Most of the SCN-resistant cultivars being grown in this region have resistance derived from a single source, Plant Introduction (PI) 88788. A survey conducted in 2005 showed that 83% of the soybean hectarage in Illinois is infested with SCN, with average population densities high enough to cause significant yield suppression (2,700 eggs/100 cm3 soil). Further characterization of these populations showed that 70% have adapted to PI 88788 at some level, reducing the effectiveness of using SCN-resistant cultivars as a crop management tool. Rotation with alternative sources of resistance is recommended as a means to slow the adaptation to PI 88788. Accepted for publication 11 October 2007. Published 18 January 2008.

Mean on‐farm U.S. soybean [Glycine max (L.) Merr.] yield increased at a rate of 23.4 kg ha−1 yr−1 between 1924 and 2010 due to a combination of genetic improvements, agronomic technologies, and climatic changes. To estimate annual rates of genetic yield gain in three northern U.S. soybean maturity groups (MGs) and determine if these estimates are influenced by cropping history, 45 MG II, 40 MG III, and 45 MG IV cultivars released between 1923 and 2008 were evaluated in split‐plot trials conducted in Illinois in 2010 where the main plot prior cropping treatments were either 11 yr of continuous corn (Zea mays L.) or 11 yr of a soybean–corn rotation. The experiment‐wide genetic yield gain estimate was 22.8 kg ha−1 yr−1 and, after covariate adjustment of yields for maturity, the estimate was 19.8 kg ha−1 yr−1. These estimates show that soybean genetic yield potential has been a large contributor to the rate of on‐farm yield improvement. The rate of yield gain estimates were not significantly different (P = 0.38) between the two cropping history treatments on an experiment‐wide basis or at the MG III and MG IV individual locations but were significantly different at the MG II locations, where yield gain for soybean following continuous corn was significantly greater compared with the soybean–corn rotation treatment. Modern cultivars were not observed to be able to close the yield gap between the two cropping history treatments used in this experiment.

The soybean [Glycine max (L.) Merr.] germplasm line LD00‐2817P (Reg. No. GP‐367, PI 658519) was developed at the University of Illinois Agricultural Experiment Station at Urbana, IL. The objective of the breeding effort that resulted in this release was the combining of high yield with soybean cyst nematode (SCN) (Heterodera glycines Ichinohe) resistance from underutilized resistance sources. LD00‐2817P was selected from the cross ‘Ina’ × ‘Dwight’ as a F4:5 line. The line was tested for yield and other agronomic traits in 56 field environments and for resistance to 14 SCN populations in inoculated greenhouse tests. The line was released because it combines high yield performance with SCN resistance originating from plant introduction (PI) 437654. This PI 437654 source confers resistance to many SCN HG Types, especially HG Type 2.5.7 (equivalent to Races 1 and 5), which is becoming more common in the midwestern USA. LD00‐2817P has a relative maturity of 4.5 and is being released as a germplasm line because of its usefulness as a parent for soybean breeders developing new cultivars with HG Type 2.5.7 resistance.

Soybean cultivars with different sources of resistance to Heterodera glycines were grown at three locations initially infested with races 2, 3, and 6 in order to investigate H. glycines race shift in field populations. Each spring and fall, soil samples were taken from each plot and race tests were conducted to evaluate effects of cultivar and time of sampling. Field experiments were paired field plots rotated annually with corn since 1991. Cultivars included at the northern and central Missouri sites were Williams 82 (susceptible to H. glycines), Linford (PI 88788 source of resistance), MFA 9043 (Peking) replaced by Morsoy 9345 (Peking and PI 88788) from 1995 to 1997, and Jackson II (Peking + PI 88788) replaced by Asgrow 3431 (Peking and PI 88788) in 1996-97. Cultivars at the southern Missouri site were Essex or Hutcheson (susceptible to H. glycines), Forrest (Peking), Hartwig (PI 437654), and Rhodes (PI 88788 + Peking). In 1995, race tests were performed at four temperature regimes to determine temperature effects on race designations. Race shifts were not predictable based on the source of resistance of the soybean cultivar planted. Variability in female numbers on Lee 74 among tests caused changes in female indices (FI). Furthermore, race designations were influenced by the time of sampling and temperature at which the race tests were conducted. The variability of H. glycines populations in both field and greenhouse situations diminishes the value of race test results when making cultivar recommendations.